Frequently it is desired in signal processing systems to multiply a signal by a sinusoid. A particular example of this is synchronous demodulation where an amplitude modulated carrier is multiplied by a reference signal corresponding to the carrier frequency in order to recover the modulating signal. To perform this operation on pulse code modulated (PCM) signals, heretofore it has been necessary to develop an analog reference signal, convert the reference signal to PCM format, and then multiply the PCM amplitude modulated carrier by the PCM reference signal. This apparatus requires at least an analog-to-digital converter and a PCM multiplier, both of which are relatively complex circuits.
Multiplication of analog sampled data signals is difficult for the reason that ideally linear analog multipliers are not practically available for consumer applications. For this reason multiplication of analog signals by a sinusoidal signal is approximated with a switching system which changes the polarity of the analog signal responsive to alternate half cycles of a square wave. This technique is generally acceptable for synchronous demodulation, however, it may introduce undesirable signal components in the demodulated signal if the signal being multiplied contains noise components at odd harmonics of the square wave frequency.
Minoru Ogita, in U.S. Pat. No. 4,404,430, disclosed apparatus which improved analog switching multipliers. The Ogita apparatus includes a voltage divider having an input terminal to which the signal to be multiplied is coupled and having a plurality of output taps from which successively attenuated signals are available. The output taps are applied to respective input terminals of a multiplexer. The attenuated signals are multiplexed to an output connection in a reciprocating manner which alternately scans through the attenuated signals in ascending order of attenuation for each first half cycle of the sinusoid and then in descending order of attenuation for alternate half cycles of the sinusoid. The accuracy of performance of this system according to Ogita increases with increasing numbers of output taps on the voltage divider.
It is an object of the present invention to provide parts efficient systems for multiplying sampled data signals by sinusoidal signals.
It is a further object of the present invention to provide a sinusoidal signal multiplying apparatus which produces products of high accuracy.